1. Field of the Invention
The present invention relates to liquid crystal displays. More specifically, it relates to reducing flicker in liquid crystal displays.
2. Discussion of the Related Art
Producing a color image using a Liquid Crystal Display (LCD) is well known. Such displays are particularly useful for producing images that are updated by frames, such as in color televisions. Typically, each image frame is composed of color sub-frames, usually red, green and blue sub-frames.
Such LCD systems employ a light crystal light panel that is comprised of a large number of individual liquid crystal pixel elements. Those pixel elements are beneficially organized in a matrix comprised of pixel rows and pixel columns. To produce a desired image, the individual pixel elements are modulated in accordance with image information. Typically, the image information is applied to the individual pixel elements by rows, with each pixel row being addressed in each frame period.
Pixel element matrix arrays are preferably xe2x80x9cactivexe2x80x9d in that each pixel element is connected to an active switching element of a matrix of such switching elements. One particularly useful active matrix liquid crystal display is the reflective active-matrix liquid crystal display (RLCD). An RLCD display is typically produced on a silicon substrate and is often based on the twisted nematic (TN) effect. Thin film transistors (TFTs) are usually used as the active switching elements. Such RLCD displays can support a high pixel density because the TFTs and their interconnections can be integrated onto the silicon substrate.
FIG. 1 schematically illustrates a single pixel element 10 of a typical RLCD. The pixel element 10 is comprised of a twisted nematic liquid crystal layer 12 that is disposed between a transparent electrode 14 and a pixel electrode 16. For convenience, FIG. 1 shows the transparent electrode applied to a common ground. Additionally, a storage element 18 is connected to complementary data terminals 20 and 22. The storage element receives control signals on a control terminal 24. In responsive to a xe2x80x9cwritexe2x80x9d control signal the storage element 18 selectively latches the data signal on one of the data terminals 20 and 22, and applies that latched signal to the pixel electrode 16 via a signal line 26. The data signals on the data terminals 20 and 22 are complementary. That is, when one line is at +2 volts, the other is at xe2x88x922 volts.
Still referring to FIG. 1, and as explained in more detail subsequently, the liquid crystal layer 12 rotates the polarization of the light 30, with the amount of polarization rotation dependent on the voltage across the liquid crystal layer 12. Ideally, the pixel element 10 is symmetrical in that the polarization rotation depends only on the magnitude of the latched signal on the signal line 26. By alternating complementary signals in consecutive frames, unwanted charges across the liquid crystal layer 12 are prevented. If only one polarity was used, ions would build up across the capacitance formed by the transparent electrode 14, the liquid crystal layer 12, and the pixel electrode 16. Such charges would bias the pixel element 10.
The light 30 is derived from incident non-polarized light 32 from an external light source (which is not shown). The non-polarized light is polarized by a first polarizer 34 to form the light 30. The light 30 passes through the transparent electrode 14, through the liquid crystal layer 12, reflects off the pixel electrode 16, passes back through the liquid crystal layer 12, passes out of the transparent electrode 14, and then is directed onto a second polarizer 36. During the double pass through the liquid crystal layer 12 the polarization of the light beam is rotated in accord with the magnitude of the voltage on the signal line 26. Only the portion of the light 30 that is parallel with the polarization direction of the second polarizer 36 passes through that polarizer. Since the passed portion depends on the amount of polarization rotation, which in turn depends on the voltage on the signal line 26, the voltage on the signal line controls the intensity of the light that leaves the pixel element.
The storage element 18 is typically a capacitor connected to a thin film transistor switch. When a control signal is applied to the gate electrode of the thin film transistor that transistor turns on. Then, the voltage applied to the source of the thin film transistor passes through the thin film transistor and charges the capacitor. When the control signal is removed, the thin film transistor opens and the capacitor potential is stored on the pixel electrode 16.
FIG. 2 schematically illustrates a pixel element matrix. As shown, a plurality of pixel elements 10, each having an associated switching thin film transistor and a storage capacitor, are arranged in a matrix of rows (horizontal) and columns (vertical). For simplicity, only a small portion of a matrix array is shown. In practice there are numerous rows, say 1290, and numerous columns, say 1024. Referring to FIG. 2, the pixel elements of a row are selected together by applying a gate (switch) control signal on a gate line, specifically the gate lines 40a, 40b, and 40c. A constant voltage (which is shared by all of the pixel elements) is applied to the transparent electrode 14 from a ramp source 41 via a line 42. Furthermore, the ramp source 41 applies complementary ramp signals on lines 20 and 22 (which are also shared by all of the pixel elements 10). Furthermore, column select lines 46a, 46b, and 46c, control the operation of the pixel elements 10.
In practice, a row of pixel elements is selected by the application of a signal on an appropriate one of the gate lines 40a-40c. This turns on all of the pixel elements in that row. Then, the ramp source 41 applies a ramp to either line 20 or line 22 (which line is used is varied in each frame). The ramp begins charging all of the storage capacitors in the selected row. As the other rows are not energized, the ramp source only charges the OFF-state capacitance of the other pixels. When the ramp voltage reaches the desired state for a particular pixel, the column select line (46a-46c) voltage for that particular pixel element 10 turns the pixel switch OFF. Then, the ramp voltage that existed when the particular pixel element 10 was turned OFF is stored on that element""s storage capacitor. Meanwhile, the ramp voltage continues to increase until all of the column select lines (46a-46c) cause a ramp voltage to be HELD on an associated pixel element. After that, a new row of pixel elements is selected and the process starts over. After all rows have been selected, the process starts over again in a new frame period, this time using the complement of the previous ramp.
The foregoing process is generally well known and is typically performed using shift registers, microcontrollers, and ramp generators. In practice, Operational Transconductance Amplifier (OTA) ramp sources are commonly used. Reasons for this include the wide dynamic range and operational bandwidths of OTAs. An OTA is a current controlled resistance amplifier that is similar to operational amplifiers, except that an OTA uses differential input current to control an output, rather than a differential voltage. Thus, an OTA includes differential inputs.
While RLCD displays are generally successful, they have problems. For example, driving a row of pixel elements using a relatively slowly changing voltage ramp, and then rapidly discharging that ramp to prepare for driving the next row of pixel elements can lead to various problems, including ramp overshoot and high power dissipation. To understand these problems, consider a ramp signal applied to an RLCD display. To a first order, the RLCD display can be modeled as a capacitance CRLCD+. The + designates that pixel elements are switched on such that current must flow into the storage elements. The OTA that produces the ramp must have a minimum slew-rate of:
SRRLCD+=xcex1(Vmaxrampxe2x88x92Vminramp)tramp 
which leads to the required OTA current of:
IRLCD+=(SRRLCD+)(CRLCD+) 
where:
SRRLCD+ is the required slew rate when charging the storage elements of a row;
xcex1 is an amplification factor determined by the required gain;
Vmaxxe2x80x94ramp is the maximum ramp voltage;
Vminxe2x80x94ramp is the minimum ramp voltage;
tramp is the ramp up time;
IRLCD+ is the maximum charging current during ramp up;
SRRLCD+ is the minimum required slew rate during ramp up; and
CRLCD+ is the maximum capacitance of the RLCD during ramp up.
Now, consider the minimum required ramp slew-rate when discharging the ramp to prepare for the next row of pixel elements:
SRRLCDxe2x88x92=xcex1(Vmaxxe2x80x94rampxe2x88x92Vminxe2x80x94ramp)tfb, 
which leads to the required OTA current of:
IRLCDxe2x88x92=(SRRLCDxe2x88x92)(CRLCDxe2x88x92) 
where:
thexe2x88x92 designates that current is not flowing into the storage elements;
SRRLCDxe2x88x92 is the required slew rate when discharging the ramp;
tfb is the ramp down (fly back) time;
IRLCDxe2x88x92 is the maximum charging (discharge) current during fly back;
SRRLCDxe2x88x92 is the minimum required slew rate during fly back; and
CRLCDxe2x88x92 is the maximum capacitance of the RLCD during fly back.
Since tramp is  greater than  greater than tfb, there is a general requirement that SRRLCDxe2x88x92 greater than  greater than SRRLCD+. To accommodate the faster slew rate (SRRLCDxe2x88x92), the ramp source is usually designed to handle the greater signal slew (and thus current) over the entire ramp cycle, in spite of the fact that the greater current handling capability is only needed for a relatively small part of each ramp cycle. This leads to high power dissipation and to inherent instabilities (such as ramp signal overshoot).
Therefore, a ramp source having a faster slew rate during ramp fly back would be beneficial. Even more beneficial would be a ramp source having a slew rate under the control of an external signal. Still more beneficial would be an OTA circuit having a slew rate controlled by an external signal. Such ramp sources and circuits would be particularly useful in liquid crystal display devices.
Accordingly, the principles of the present invention provide for operational transconductance amplifier (OTA) circuits having slew rates controlled by external signals, for ramp sources having increased slew capability during ramp fly back, and for liquid crystal display devices having ramp sources with controlled slew rates.
A circuit according to the principles of the present invention includes an operational transconductance amplifier (OTA). The OTA has an output that drives a load, which is beneficially a liquid crystal display panel. The OTA circuit includes a first current source that sinks a first tail current, and a second current source that selectively sinks a second tail current. The second current source is selected by a control signal applied to a switch. When the second current source sinks the additional second tail current the slew rate of the OTA is increased. When used as a ramp source, the OTA circuit receives a ramp input that is applied to the non-inverting input of the OTA. Then, the slew rate is increased during ramp fly back. When used in a liquid crystal display, the OTA circuit drives a liquid crystal display panel with a ramp such that the ramp""s slew rate is increased during fly back.
Additional features and advantages of the invention will be set forth in the description that follows, and in part will be apparent from that description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.